Is it possible to replicate DNS between the two? Obviously they have network access between each other.. But how do I make it so the DNS servers in DomainA replicate any / all changes to DNS servers in DomainB and vice/versa?

You are better of chaining them otherwise you could get redirect loops.
example: Address not found at server A redirects to server B and if not found there redirect to server C if not found redirect to google DNS
The way you have it it will get to server C and redirect to server A never resolving to anything
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If the zones are small and the link between the servers for the 2 domains is stable you may find it easier to simply have forwarders. This way if the servers in domain A received a lookup request for some resource in domain B it forwards the requests to the servers in B. Obviously this lookup will fail if the server in B is unavailable whereas with a secondary lookup zone the lookup would still succeed because the server in A has a copy of the zone for domain B.

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What are you trying to accomplish? Lets say you have domains A.COM and B.COM. You aren't going to be able to replicate so that if you created a record for SERVER1.A.COM a record replicates over as SERVER1.B.COM with the same lookup value.

You could created a Secondary Forward Lookup Zone and that will repliate the DNS zone from the authoritative A.COM and keep a copy of the zone on B.COM's DNS servers so that B.COM's servers can resolve for your clients. Depending on your size between the 2 zones and the amount of traffic you may be better off with a forward lookup stub zone or even just forwarders.

If the zones are small and the link between the servers for the 2 domains is stable you may find it easier to simply have forwarders. This way if the servers in domain A received a lookup request for some resource in domain B it forwards the requests to the servers in B. Obviously this lookup will fail if the server in B is unavailable whereas with a secondary lookup zone the lookup would still succeed because the server in A has a copy of the zone for domain B.

1st Post

<<I know I can put a DNS server for DomainA and a DNS server for DomainB
in the client's IP settings so they can resolve both, which is what I
may have to do if this isn't possible.>>

NOT TRUE!!!!

That is NOT how DNS works. A client EITHER uses DNS server A OR DNS server B. If a record is not found in DNS sever A the client will NOT look in DNS server B. In fact, the only time the client will attempt to switch which DNS server it uses is if the DNS server is completely unavailable (both tcp and icmp).

The only way this works is if the DNS servers forward or replicate the settings between themselves.